% !TeX program = xelatex
% !TeX encoding = UTF-8
\documentclass[UTF8]{standalone}
\usepackage{amsmath,fourier,ctex,tikz}
\begin{document}
\begin{tikzpicture}
	\draw[-latex] (0,0) node[below] {$O$} -- ++ (0,2.5) node[right] {$z$};
	\draw[-latex] (0,0) -- ++ (2.5,0) node[below] {$y$};
	\draw[-latex] (0,0) -- ++ (-135:2.5) node[right] {$x$};
	\draw (0,1) circle (1);
	\draw[dashed] (1,1) arc [start angle=0, end angle=180, x radius=1, y radius=0.4];
	\draw (-1,1) arc [start angle=180, end angle=360, x radius=1, y radius=0.4];
	\draw[dashed] (0,1) -- node[below] {$a$} ++ (1,0);
\end{tikzpicture}
\end{document}